netdev - Re: [PATCH net-next 0/1] Add BASE-T1 PHY support

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Date:   Mon, 19 Aug 2019 21:07:22 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Christian Herber <christian.herber@....com>,
        "davem@...emloft.net" <davem@...emloft.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 0/1] Add BASE-T1 PHY support

On 19.08.2019 08:32, Christian Herber wrote:
> On 16.08.2019 22:59, Heiner Kallweit wrote:
>> On 15.08.2019 17:32, Christian Herber wrote:
>>> This patch adds basic support for BASE-T1 PHYs in the framework.
>>> BASE-T1 PHYs main area of application are automotive and industrial.
>>> BASE-T1 is standardized in IEEE 802.3, namely
>>> - IEEE 802.3bw: 100BASE-T1
>>> - IEEE 802.3bp 1000BASE-T1
>>> - IEEE 802.3cg: 10BASE-T1L and 10BASE-T1S
>>>
>>> There are no products which contain BASE-T1 and consumer type PHYs like
>>> 1000BASE-T. However, devices exist which combine 100BASE-T1 and 1000BASE-T1
>>> PHYs with auto-negotiation.
>>
>> Is this meant in a way that *currently* there are no PHY's combining Base-T1
>> with normal Base-T modes? Or are there reasons why this isn't possible in
>> general? I'm asking because we have PHY's combining copper and fiber, and e.g.
>> the mentioned Aquantia PHY that combines NBase-T with 1000Base-T2.
>>
>>>
>>> The intention of this patch is to make use of the existing Clause 45 functions.
>>> BASE-T1 adds some additional registers e.g. for aneg control, which follow a
>>> similiar register layout as the existing devices. The bits which are used in
>>> BASE-T1 specific registers are the same as in basic registers, thus the
>>> existing functions can be resued, with get_aneg_ctrl() selecting the correct
>>> register address.
>>>
>> If Base-T1 can't be combined with other modes then at a first glance I see no
>> benefit in defining new registers e.g. for aneg control, and the standard ones
>> are unused. Why not using the standard registers? Can you shed some light on that?
>>
>> Are the new registers internally shadowed to the standard location?
>> That's something I've seen on other PHY's: one register appears in different
>> places in different devices.
>>
>>> The current version of ethtool has been prepared for 100/1000BASE-T1 and works
>>> with this patch. 10BASE-T1 needs to be added to ethtool.
>>>
>>> Christian Herber (1):
>>>    Added BASE-T1 PHY support to PHY Subsystem
>>>
>>>   drivers/net/phy/phy-c45.c    | 113 +++++++++++++++++++++++++++++++----
>>>   drivers/net/phy/phy-core.c   |   4 +-
>>>   include/uapi/linux/ethtool.h |   2 +
>>>   include/uapi/linux/mdio.h    |  21 +++++++
>>>   4 files changed, 129 insertions(+), 11 deletions(-)
>>>
>>
>> Heiner
>>
> 
> Hi Heiner,
> 
> I do not think the Aquantia part you are describing is publicly 
> documented, so i cannot comment on that part.
Right, datasheet isn't publicly available. All I wanted to say with
mentioning this PHY: It's not a rare exception that a PHY combines
standard BaseT modes with "non-consumer" modes for special purposes.
One practical use case of this proprietary 1000Base-T2 mode is
re-using existing 2-pair cabling in aircrafts.

> There are multiple reasons why e.g. xBASE-T1 plus 1000BASE-T is 
> unlikely. First, the is no use-case known to me, where this would be 
> required. Second, there is no way that you can do an auto-negotiation 
> between the two, as these both have their own auto-neg defined (Clause 
> 28/73 vs. Clause 98). Thirdly, if you would ever have a product with 
> both, I believe it would just include two full PHYs and a way to select 
> which flavor you want. Of course, this is the theory until proven 
> otherwise, but to me it is sufficient to use a single driver.
> 
I'm with you if you say it's unlikely. However your statement in the
commit message leaves the impression that there can't be such a device.
And that's a difference.

Regarding "including two full PHYs":
This case we have already, there are PHYs combining different IP blocks,
each one supporting a specific mode (e.g. copper and fiber). There you
also have the case of different autoneg methods, clause 28 vs. clause 37.

> The registers are different in the fields they include. It is just that 
> the flags which are used by the Linux driver, like restarting auto-neg, 
> are at the same position.
> 
Good to know. Your commit description doesn't mention any specific PHY.
I suppose you have PHYs you'd like to operate with the genphy_c45 driver.
Could you give an example? And ideally, is a public datasheet available?

> Christian
> 
> 
Heiner